International Mobile Telecommunication (IMT)-12000 systems aim to solve the problem of global roaming between mobile communication systems, and are classified into a Code Division Multiple Access (CDMA) 2000-based synchronous system and a Wideband Code Division Multiple Access (WCDMA)-based asynchronous system.
A CDMA2000-based synchronous mobile communication system is implemented to cover a wide area, and is advantageous in that a maximal data rate of 2.4 Mbps can be obtained in a forward direction when an Evolution Data Only (EV-DO) scheme is applied thereto. However, since the data rate varies greatly with the wireless environment, the CDMA-based synchronous mobile communication system is disadvantageous in that it is not appropriate for services, such as a video phone service, for which Quality of Service (QoS) must be guaranteed.
In contrast, a WCDMA-based asynchronous mobile communication system is advantageous in that it is appropriate for real-time services, such as a video phone service, because a circuit switching is supported, so that high-speed mobility is supported and the QoS thereof is excellent. However, since the investment cost is excessive at the early stage of commercialization, the WCDMA-based asynchronous mobile communication system is disadvantageous in that it is difficult to provide a service covering the entire area in which a synchronous mobile system is established.
Accordingly, in the early IMT-2000 system, a service is provided in a form in which synchronous and asynchronous mobile communication systems coexist. For this purpose, a multi-mode mobile communication terminal is required. In this case, when the synchronous mobile communication system is used for voice and high-speed Internet access services and the asynchronous mobile communication system is used for services, such as a video phone service, for which QoS must be guaranteed, the advantages of both systems can be maximized.
Meanwhile, a multi-mode mobile communication terminal for supporting these advantages, that is, a Dual Band Dual Mode (DBDM) mobile communication terminal that can be used both in a synchronous type system and in an asynchronous type system has been developed. By using the mobile communication terminal, different services can be used in a synchronous system area and an asynchronous system area.
In the case of the DBDM mobile communication terminal, since the lifespan of a battery charge is greatly shortened due to considerable power consumption when two modem clips, which are contained therein, are used, only one of the two modem chips should be operated. Accordingly, the DBDM mobile communication terminal is selectively connected to the synchronous mobile communication system and the asynchronous mobile communication system.
In a mobile communication environment implemented such that an asynchronous mobile communication system overlaps a synchronous mobile communication system, call drops may occur in a border region between synchronous and asynchronous areas. Furthermore, in the case where a user moves between a synchronous mobile communication system and an asynchronous mobile communication system, handover between the systems is required to provide continuous service. However, a handover method between asynchronous and synchronous mobile communication systems has not yet been defined. If the handover method is not defined, there occurs a disadvantage in that vast costs are required for the implementation of the asynchronous mobile communication system.
Furthermore, when a mobile communication terminal is located in the border region between synchronous and asynchronous areas even though handover is supported, it must be possible to search for a synchronous mobile communication system. This case is problematic in that, since the two modem chips contained in the mobile communication terminal must both operate, interference may be induced therebetween, and the power consumption of the mobile communication terminal is increased.